In zirconium oxide (IV) (ZrO2) (referred to as “zirconia” hereinafter), there are polymorphs, and phase transition occurs between the polymorphs. For example, tetragonal zirconia changes to monoclinic zirconia by the phase transition. Therefore, a tetragonal zirconia sintered body in which the phase transition is not suppressed does not have enough strength to be used as a product. The phase transition also causes volume change. Therefore, the tetragonal zirconia sintered body in which the phase transition is not suppressed can not be applied to a product having high accuracy of a size. The phase transition further causes lowering of durability of the zirconia sintered body.
Therefore, oxides such as yttrium oxide (Y2O3) (referred to as “yttria” hereinafter) are used as a stabilizer for suppressing the occurrence of the phase transition. The tetragonal zirconia which is partially stabilized by adding the stabilizer has been referred to as partially-stabilized zirconia (PSZ) and has been used in various fields. A sintered body of the partially-stabilized zirconia (referred to as “zirconia sintered body” hereinafter) has been used for dental prostheses, tools, etc.
Patent Literatures 1-3 disclose a zirconia sintered body that is especially used for dentistry. The zirconia sintered bodies disclosed in Patent literatures 1-3 contain 2-4 mol % yttria as a stabilizer, and have a relative density of 99.8% or higher and a total light transparence, as measured at a thickness of 1.0 mm, of 35% or higher.
Patent Literature 4 discloses a zirconia sintered body that contains one or more of yttria, calcia, magnesia and ceria; containing one or more kind(s) of a positive ion having a smaller ionic radius than that of a zirconium ion and/or a positive ion except for that having valence of 4; a rate of a monoclinic phase after immersed into hot water at a temperature of 140 degrees Celsius for 72 hours being 1% or less.
Patent Literature 5 discloses a zirconia sintered body that has partially-stabilized zirconia, as a matrix phase, including a stabilizer, wherein a standard deviation of a surface content of the stabilizer is 0.8 or higher when a region of 10 μm by 10 μm in a sample surface of the zirconia sintered body is divided into squares of 256 by 256, and the content of the stabilizer in each of the squares is expressed in wt %.    PATENT LITERATURE 1: JP Patent Kokai Publication No. JP2009-269812A    PATENT LITERATURE 2: JP Patent Kokai Publication No. JP2010-150063A    PATENT LITERATURE 3: JP Patent Kokai Publication No. JP2010-150064A    PATENT LITERATURE 4: JP Patent Kokai Publication No. JP2007-332026A    PATENT LITERATURE 5: JP Patent Kokai Publication No. JP2011-178610A